Gradient methods applied to simulated ULF data: The effects of the ionospheric damping factor

The gradient methods have widely been used to identify the field‐line resonance (FLR) signature in the ULF range in data from two latitudinally separated ground magnetometers. However, there are also cases in which ULF waves exist in the data but the gradient methods do not identify an FLR signature; in such cases, it is unclear if FLR is actually nonexistent or if some other factors bias the gradient‐method output. As a step toward finding a way to distinguish these two, in this paper we apply, for the first time, the gradient methods to MHD‐simulation results to examine the gradient methods. As a first step, we have confirmed that there are cases in which the gradient methods successfully identify FLR. We have then applied the gradient methods to the results of two simulation runs with different ionospheric reflection coefficients for the impinging wave (100 and 60%). As a result, we have found that the overhead ionospheric reflection coefficient (controlled by the ionospheric conductivity) affects the proper spacing of the two ground magnetometers; an improper spacing is a possible reason for the gradient method's being unsuccessful. We have also found that, when the ionosphere is not a perfect reflector, which is the case in reality, a non‐FLR wave component biases the gradient‐method output; the non‐FLR component appears to arise from the coupling of an FLR component and a cavity‐mode component where the two frequencies match. This coupling is another possible reason that the gradient methods are unsuccessful.